It has been recognized for some time that protein-providing feed materials which are subject to digestion in the rumen are thereby, in effect, downgraded with respect to the feeding value of the protein. It has been proposed that ideally the protein component of the ruminant feed should be "protected" against being solubilized or metabolized in the rumen, passing therethrough in substantially undegraded form, while remaining digestable and metabolizable in the post-rumen digestive system of the cattle or sheep. The development of a practical way for applying this concept to ruminate nutrition has proven difficult.
U.S. Pat. No. 3,619,200 proposes the application to the vegetable meal or other proteinaceous ruminant feed material of a rumen resistant coating. The purpose of the coating is to protect the proteinaceous feed from microbial attack in the rumen while decomposing and permitting digestion of the feed within the abomasum and small intestine. It is also known that the solubility of protein in ruminant feed materials can be reduced by treating the feed materials with tannin, formaldehyde, or other aldehydes. In addition, a reduction in protein solubility can be obtained by heating the protein. These procedures are summarized with literature references thereto in U.S. Pat. No. 4,186,213. Feed materials which may be treated by one or more of these procedures to reduce the solubility of the protein in the rumen and to protect against rumen destruction include particularly soybean meal and other oil seed meals.
With reference to feeding value lost by rumen destruction, soybean meal has a relatively low protein efficiency value. See Klopfenstein, Feedstuffs, July, 1981, 23-24. Since soybean meal is one of the major protein-containing feed materials used with cattle, it is particularly desirable to provide a commercially practical means for protecting soybean meal against rumen destruction while leaving the protein thereof subject to post-rumen digestion and metabolism. For large scale commercial use such a method must be simple, efficient, and of relatively low cost.
I have previously disclosed that water-soluble zinc salts, such as zinc sulfate and zinc chloride, can be combined with proteinaceous feeds for ruminants to improve protein utilization. In one procedure, which I initially preferred, the water-soluble zinc salt is prereacted with the proteinaceous feed materials. (See my published European Patent Application No. 0 107 749.) As described therein, the zinc salt may be applied to the feed material as an aqueous solution and heated in contact therewith. Alternatively, the zinc salt may be dry mixed with the feed materials and then subjected to pelleting, which involves the application of heat in the presence of moisture.
Subsequently, it was found that the pelleting of the zinc salt-containing dry blend was not essential, and that a useful degree of protection of the protein could be obtained by feeding the dry blend containing the water-soluble zinc salt in an unreacted particulate form. (See my South African Patent No. 85/0134.)
Since the zinc in the blend is in the form of a highly water-soluble salt, it was speculated that zinc ions were rapidly formed in the saliva and in the rumen by dissolving of the zinc salt, and that the zinc ions reacted quickly enough with the protein of the feed material to provide effective rumen protection. However, the mechanism action of the zinc salt in the rumen has no been established.
Zinc compounds of limited water solubility such a zinc oxide and zinc carbonate have heretofore been employed for supplying micronutrient zinc to ruminants. As far as is known, no study has been published on the action of rumen fluids on zinc oxide or other zinc compounds of low water solubility. The utilization of zinc as a micronutrient is post-ruminal, and the amount of zinc required as a micronutrient for cattle is quite small, typically not over 50 ppm based on the total daily diet. Small amounts of zinc are present in cattle feed materials, such as proteinaceous oil seed meals, ranging from 25 to 150 ppm. Lease and Williams, Poultry Science, 46:233-241, Table 1 at 2364 (1967).
The toxicity of high levels of zinc oxide has been studied: Miller et al., J. Dairy Sci., 48:450-453 (1966); Ott, et al., J. Anim. Sci , 25:414-438 (1966). These investigators tested zinc oxide in admixture with ruminant feed materials in amounts greater than micronutrient levels. Miller, et al. fed amounts of zinc to lactating dairy cattle up to 2000 ppm based on the feed concentrate (1279 ppm for total diet). No beneficial effect on milk production was observed by the increased ZnO in the diet. Ott, et al. fed lambs pelleted rations containing from 0.5 to 4.0 grams of zinc per kilogram of diet, and beef cattle from 1.0 to 3.0 grams zinc per kilogram of diet. These rations included soybean meal. Lambs receiving 0.5 to 1.0 grams zinc gained somewhat faster than the controls, but no increase in rate of weight gain was observed for the beef cattle. Ott, et al. reported that steers were less adversely affected by high levels of zinc than heifers.